1. Field of the Invention
This invention relates to methods for seismic exploration, particularly to methods for seismic exploration using multiple impulse or long emission seismic signals, and most particularly to methods for improving the quality of the seismic traces obtained in seismic exploration methods.
2. Description of the Prior Art
Seismic exploration techniques, in which energy in the form of seismic waves is transmitted into the earth and in which the reflected waves are detected, converted to digital signals, and recorded, are well known in the art. In very early methods, a relatively large explosive charge was detonated at, or just below, the surface of the earth. Due to the large amount of energy transmitted into the ground, the reflected energy waves were of sufficient amplitude as to be easily distinguishable from the random background noise which also appeared in the recorded seismic trace. More recently, methods have been developed in which a plurality of relatively small energy emissions or waves are transmitted into the earth. In these methods, the recorded traces from numerous emissions are "crosscorrelated" and "stacked" to produce a finished trace of satisfactory quality. The success of these methods is attributable to the fact that background noise is relatively random and, therefore, as the numerous traces are stacked, the recorded waves attributable to this noise are out of phase and will therefore on adding be effectively cancelled. The waves due to actual seismic events, on the other hand, will add in phase and be reinforced, thereby building a high ratio of reflective signal-to-noise amplitude.
There are basically three techniques of seismic exploration with low energy multiple emission sources. These techniques are distinguished by their respective seismic source devices and the timing of the source emissions as follows:
(1) "Single shot-listen" techniques, such as those disclosed in U.S. Pat. No. 3,956,730 to Barbier, in which a single impulse from a small explosive charge or a weight-dropping apparatus is transmitted into the earth and the waves reflected from within the earth are recorded during a "listening period", which is at least as long as the two-way travel time to the deepest reflector of interest, before the next source impulse is transmitted; PA1 (2) "Long source-listen" techniques, such as the well known VIBROSEIS.RTM. system, in which a single relatively long, amplitude and/or frequency varying signal is transmitted into the earth and the waves reflected from within the earth are recorded during a "listening period", which is at least as long as the source emission time plus the two-way travel time after the end of the long signal, before the next long signal is transmitted; PA1 (3) "Impulse train" techniques, such as disclosed in U.S. Pat. Nos. 3,622,970 to Sayous et al. and 3,698,009 to Barbier, in which a plurality of nearly identical impulses are transmitted into the earth, usually according to a precise code. In the code, the individual impulses are separated by varying time intervals of less than the two-way travel time but the duration of the impulse train is longer than the two-way travel time. The waves reflected from within the earth are recorded for a period of time usually at least equal to the duration of the impulse train plus the two-way travel time before the next impulse train is transmitted. PA1 a. vertical stack--individual traces with nearly identical source and geophone locations are "stacked" (added) together usually without correction for moveout; PA1 b. crosscorrelation--the vertically stacked traces are correlated with a record of the source code; PA1 c. moveout correction--the reflected events of the trace are time shifted to correct for each of the three types of moveout (normal, dip, and static); and PA1 d. common depth point (CDP) stack--traces from sets of sources and geophones which have a common midpoint between the respective source and geophone are stacked.
The subsequent processing of the individual traces from each of these techniques is similar and is well known in the art. If a series of raw traces are obtained by repeated emission of a single source code, the traces are conventionally processed as follows:
Optionally, various conventional filtering and/or muting methods may be applied to the traces. Subsequently, the reflection travel times from these traces are converted to depths and are then compiled to form depth maps and cross sections of the earth strata explored.
If, however, the series of raw traces had been obtained by emission of several different coded energy signals, the order of steps a and b above would be reversed, i.e., each trace would be crosscorrelated with a signature of the corresponding coded energy signal before vertical stacking.
The primary purpose of these processing techniques is to increase the reflective signal-to-background noise amplitude ratio. However, in the crosscorrelation step, a different type of noise, in which the peaks are commonly known as correlation residuals or side lobes, is introduced into the seismic trace. Since these residuals broaden the time duration of the seismic events thereby decreasing seismic resolution, and since they are of no seismic interest and indeed have no physical significance, they are as undesirable as the naturally occurring background noises.
Various methods have been devised to reduce the size of the correlation residuals and thereby increase the ratio of reflective event signal-to-noise (background plus residual) amplitude. U.S. Pat. No. 3,326,320 to Forester discloses a seismic survey method in which two specially coded source signals are separately transmitted into the earth from the same sourcepoint and sensor locations and the reflected seismic trace from the emission of the second code is subtracted from the trace of the first code emission. U.S. Pat. No. 3,622,970 to Sayous et al. discloses a seismic exploration method in which a specially coded impulse train of constant amplitude and polarity impulses is transmitted into the earth and the resulting seismic trace is crosscorrelated by the shift-summing method. While these and other prior art methods can reduce the relative amplitude of the correlation residuals, they require precisely controlled, and usually expensive, seismic sources. Furthermore, even though the residuals of these methods are ideally only 1/10 to 1/20 of the amplitude of the reflective event, the residuals can still obscure the reflected energy from relatively weak seismic events. This interference is especially detrimental in very deep surveying because the energy reflected from even major, deep reflectors is relatively weak.
Accordingly, a primary object of this invention is to provide an improved seismic exploration method.
Another object of this invention is to provide an improved seismic exploration method which does not require precisely controlled seismic sources.
Still another object of this invention is to provide an improved seismic exploration method resulting in the production of high quality seismic data.
Yet another object of this invention is to provide an improved method for delineating subterranean mineral deposits using reflective seismic exploration.
A further object of this invention is to provide a seismic exploration method which produces highly resolved seismic traces and which thereby allows more accurate delineation of subterranean mineral deposits for the subsequent placement of wells to develop the mineral deposits.
A still further object of this invention is to improve the quality of seismic traces obtained from the various seismic exploration techniques by removing from the data the correlation residuals which are introduced during the correlation step of conventional data processing.
An additional object of this invention is to provide a method in which high quality seismic data is produced using coded energy signals which can be generated by less sophisticated seismic sources than the precisely controlled seismic sources of the prior art.
Yet another object of this invention is to improve the ratio of seismic event signal-to-noise amplitude by predictively removing the correlation residuals from the crosscorrelated seismic trace and thereby eliminate the requirement for the high quality coded energy signals which can only be generated by precisely repeatable and controllable seismic sources.
Other objects and advantages of this invention will become apparent to those skilled in the art from the following description.